Falling into water from moving ship question

[p1t1o]

So the common wisdom is that if you fall off a moving ship, you are in dire risk of being "sucked" under the water by the turbulence of its passing, or possibly drawn into the propellers.

Does anyone know if this is true?

[kerbiloid]

Bernoulli's principle

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Spoiler

 

 

Edited April 24, 2017 by kerbiloid

[Green Baron]

Hi,

there are different effects at work. Let's assume open water, that makes it easy. In a canal or a narrow things get complicated. Also a boat that leaves it's wave-system (gliding/surfing etc.) is a different thing. So, the clear answer is: it depends (where you fall in) !

Have a look at the waterline of a moving boat: there is overpressure at the bow, shown by a wave the boat pushes in front it. Water then accelerates along the hull, and according to Bernoulli (or so) the pressure sinks, as shown by a long 1/2 lambda "negative" wave along the waterline. At the stern there is underpressure and turbulence, "sucking" the boat backwards.

Additionally there is the propulsion, usually a (or more) propeller that shoves water from front to back thus pushing the hull through the water. So if you happen to drop in the first third of hull of a long boat you are probably pushed away by the water. If you happen to drop in the under pressure area where maybe the suction of the propeller helps you have a good chance of having fun in the Great Roundabout. If you hop off near the stern you'll probably just hit the hull and then be left behind, reflecting over the future and your family having fun with your life insurance.

 

Me as a hobby sailor tells you: falling overboard is almost sure death, because if no IMMEDIATE (seconds) action is taken you will not be found again. Water temperature and quality of the life vest determine how much time you have left. A "person overboard" is an emergency and reason for an immediate distress (aka "Mayday").

:-)

Edit: partly ninja'd by @kerbiloid

Second edit: i mean, of course, a quiet inland lake is another thing like a journey on the open waters.

Edited April 24, 2017 by Green Baron

[peadar1987]

Having spent most of my youth in and around small boats, both sailing and powered, I think it's unlikely you'd be dragged into the prop. I've never seen it happen. The harbour where I learned to sail (Dún Laoghaire) was a pretty busy ferry port. There was always a lot of rubbish and debris in the harbour, and none of it seemed to get chewed up by the props of the ferries passing through, or the impeller on the fast catamaran that replaced them. Obviously this is a slightly different situation to stuff falling off the ship, but stuff already in the water generally tends to get pushed aside by the bow wave before it gets anywhere near the spinning death at the stern.

One thing I have noticed is that at low speeds the boat can "tow" along quite a lot of stuff behind it. On my small sailing boat, below about 2 knots of speed, two eddies form at the corners of the stern, and small pieces of debris will sit in these vortices and follow me along.

 

[magnemoe]

It happen from time to time that people fall overboard from ships, being drunk and balance on the railing is one. 
They are not sucked into the propeller. For one the boat is far wider than the propeller size. 
On small boats things has to pass under the boat to have an chance hitting the propeller, this happen from time to time with rope or waterlogged wood. 

As Green Barron say, main problem is stopping then lower an rescue boat and then finding them. 

[Green Baron]

Yes, the effect is for an ocean ship moving at some speed. A maneuvering or slow moving ferry or a small sailing boat does not have an expressed wave system. The small sailing boat because is too small and usually doesn't move at its hull speed under engine and the hull is normally very effectively streamlined. A small motorboat usually leaves it's wave system and pushes itself on the bow wave. Also the small to tiny propeller eventually might hurt you (even badly) if you get in contact with it but it will not kill you.

People falling overboard on the open waters usually are not found any more. These are more than might think, quite few every year, more from recreational boats than huge ships. People falling overboard in harbour areas have a tiny chance, if it is not by night. But often times you read or hear the SAR messages on the information systems (NAVTEX) for a few hours and then they are canceled.

If you want to learn more i'd suggest you take a look at the reports of the various marine accident investigation boards.

:-)

Edit: how strong the Bernoulli effect can actually be is best shown by accidents in narrow canals or tidal streams ways, were even huge ships can collide just because the effect attracts them until contact if not counteracted in time.

Editedit: and accidents between small boats and swimmers/divers sadly happen very often ! Normally beaches and swimming areas are separated from the open water by a chain of yellow buoys signaling that no boat traffic is allowed. Sometimes channels, marked with a red and green buoy signal a narrow path for dingies or diver boats when there is an anchorage nearby.

 

Edited April 24, 2017 by Green Baron

[tater]

Is cavitation an issue with powered ship wakes? It seems like liberating gases would lower the density of the water in the immediate wake, and make it impossible to float.

[p1t1o]

So it seems it strongly depends on the size and speed of the vessel, but it could happen (extreme in-and-of-itself risk of being overboard notwithstanding).

 

4 minutes ago, tater said:

Is cavitation an issue with powered ship wakes? It seems like liberating gases would lower the density of the water in the immediate wake, and make it impossible to float.

Cavitation bubbles collapse rapidly, but air entrained from the surface could have a similar effect.

[sevenperforce]

10 minutes ago, tater said:

Is cavitation an issue with powered ship wakes? It seems like liberating gases would lower the density of the water in the immediate wake, and make it impossible to float.

I doubt this would be an issue in a powered ship wake, but I know this is the reason why a sinking ship can cause lifeboats in its vicinity to sink. As the bulkheads rupture and other trapped air is released, it lowers the effective density of the water above it and so buoyancy fails.

No sucking vortex though.

[tater]

I haven't looked into terribly many wakes on various craft I've been on, but it seems frothy from casual observation, hence my question. Regardless, it would be pretty transient for someone overboard as that effect seems (visually) to be very close to the stern.

[sevenperforce]

28 minutes ago, tater said:

I haven't looked into terribly many wakes on various craft I've been on, but it seems frothy from casual observation, hence my question. Regardless, it would be pretty transient for someone overboard as that effect seems (visually) to be very close to the stern.

The froth is mostly microbubbles...not enough to significantly lower density.

[kerbiloid]

2 hours ago, p1t1o said:

So it seems it strongly depends on the size and speed of the vessel

On speed and size relative to distance.

Also you can make an experiment: pour a full bath of water, let something float and quickly scratch with your hand near it.

[peadar1987]

17 hours ago, tater said:

Is cavitation an issue with powered ship wakes? It seems like liberating gases would lower the density of the water in the immediate wake, and make it impossible to float.

Not really, cavitation causes parasitic drag if it happens on the hull, or reduces the efficiency of the prop if it happens there. Most ships will be designed to avoid it if possible.

[tater]

I was thinking of unavoidably lowering the density of the water in the wake, I was guessing it might be cavitation. I know nothing about the subject, just seemed interesting, and I have certainly notice frothy water in a wake.

Edited April 25, 2017 by tater

[p1t1o]

11 minutes ago, tater said:

I was thinking of unavoidably lowering the density of the water in the wake, I was guessing it might be cavitation. I know nothing about the subject, just seemed interesting, and I have certainly notice frothy water in a wake.

The density re-asserts itself quite readily though, hence the froth is on the surface where you can see it. If a large volume of water had its density lowered drastically for any length of time, it would float to the surface and spread out across it like an oil slick, which you can see, it kinda does.

Bubbles caused by cavitation - I recently said they collapse rapidly. By "rapidly" I meant "whilst still in contact with, or very near to, the surface causing them." - they can only exist in the regions of lowered pressure caused by the passage of the propeller blades (or whatever caused it). Occasionally, there may be some slightly longer lived areas of lowered pressure in vortices and the like, where some bubbles may last for a few seconds, but it wont be a great volume. The frothing you see is caused by air entrained from the atmosphere by a turbulent surface, the propellers, and possibly effects caused by the passage of the vessel.

[Green Baron]

I know the effect plays a role in river rafting and canoeing behind a step or waterfall, in combination with dynamic movement of water.

It might be that mixing of water and air by the propulsion reduces the lift capacity of water. The waters behind fast ferries that run 35+ knots don't look calm at all. Whether it's enough to drown a person, idk. On the other hand, the foam may be only superficial as engineers probably try anything to keep the propellers/jets fully submerged at all times efficiency wise ...

Edit: not cavitation, that was simply about mixing of air/water by dynamic movement. Cavitation is, as far as i know avoided as far possible. If at all it only plays a local roll at the blades of propellers, i would guess.

Edited April 25, 2017 by Green Baron

[sevenperforce]

23 minutes ago, tater said:

I was thinking of unavoidably lowering the density of the water in the wake, I was guessing it might be cavitation. I know nothing about the subject, just seemed interesting, and I have certainly notice frothy water in a wake.

Cavitation won't change the overall density of the water, not for any meaningful period of time. Entrained air might.

[p1t1o]

I do know that some propellers for high-speed applications are specifically designed to operate whilst cavitating, whilst others are designed to operate only half-submerged, to obtain the same effect by entraining atmospheric air.

https://en.wikipedia.org/wiki/Supercavitating_propeller

 

[Green Baron]

1 minute ago, p1t1o said:

I do know that some propellers for high-speed applications are specifically designed to operate whilst cavitating, whilst others are designed to operate only half-submerged, to obtain the same effect by entraining atmospheric air.

https://en.wikipedia.org/wiki/Supercavitating_propeller

 

Ok, but the passenger- and merchant ships don't rely on such things ? I read the moving parts wear off quickly. Ferries here are only taken out of the water after impact into harbour walls :-))

The fast multihulls that operate here have diesel engines and water jets.

[p1t1o]

1 minute ago, Green Baron said:

Ok, but the passenger- and merchant ships don't rely on such things ? I read the moving parts wear off quickly. Ferries here are only taken out of the water after impact into harbour walls :-))

The fast multihulls that operate here have diesel engines and water jets.

Oh of course, I think those special propellers are quite niche. I just mentioned it for completeness/interest.

[tater]

It is indeed cavitation, at least according to nature. Like I said, I knew nothing about this until just googling it (home with sick kid).

https://www.nature.com/scitable/blog/saltwater-science/ship_wakes_are_made_of

The reason the bubble density is so high in a wake is that the pressure on the trailing edge of the prop is so low that the water vaporizes, it's not entrained air from the surface. As the prop moves forward, away from the bubble so formed, the bubble implodes (this is cavitation).

Edited April 25, 2017 by tater

[peadar1987]

48 minutes ago, Green Baron said:

I know the effect plays a role in river rafting and canoeing behind a step or waterfall, in combination with dynamic movement of water.

It might be that mixing of water and air by the propulsion reduces the lift capacity of water. The waters behind fast ferries that run 35+ knots don't look calm at all. Whether it's enough to drown a person, idk. On the other hand, the foam may be only superficial as engineers probably try anything to keep the propellers/jets fully submerged at all times efficiency wise ...

Edit: not cavitation, that was simply about mixing of air/water by dynamic movement. Cavitation is, as far as i know avoided as far possible. If at all it only plays a local roll at the blades of propellers, i would guess.

I've driven a RIB pretty close behind a fast ferry (This ugly brute)

Not close enough to get hit by the jets from the impellers, but close enough that the water was still fizzing. There was no real noticeable drop in the buoyancy of the RIB, or the power from the prop when we entered the frothy water.

[tater]

(found a cool paper on acoustic propagation through ship wakes as well that was pretty interesting, lol)

Edited April 25, 2017 by tater

[Green Baron]

Sure, give it here :-)

I didn't expect the effect takes place at slow moving propellers as well. I never attributed the bubble spur behind the (plastic !) dinghi prop as cavitation. The 3 blade folding prop of my sailing boat never showed that, but max rpm was 2000/min (which i never really used).

Edited April 25, 2017 by Green Baron

[p1t1o]

7 minutes ago, tater said:

It is indeed cavitation, at least according to nature. Like I said, I knew nothing about this until just googling it (home with sick kid).

https://www.nature.com/scitable/blog/saltwater-science/ship_wakes_are_made_of

The reason the bubble density is so high in a wake is that the pressure on the trailing edge of the prop is so low that the water vaporizes, it's not entrained air from the surface. As the prop moves forward, away from the bubble so formed, the bubble implodes (this is cavitation).

That does give a good description of cavitation, but - and I am at risk of suffering from an over-inflated ego here, for sure - it doesnt explain how a slower, non-cavitating ship would produce a frothy wake. Everything about cavitation says the bubbles collapse rather rapidly, even the included explanation of cavitation damage supports that.

And you can see in images of ships, frothy foam is generated at the bow as well, thats not cavitation.

Theres plenty I dont know about the subject though.

 

"These cavitation bubbles have effects in addition to forming a bubble trail behind the ship. First of all, each mini implosion makes a loud noise."

These two sentences seem incompatible with each other?